1) Field of the Invention
The present invention relates to an optical receiving apparatus and optical level adjusted quantity setting method used therefore, suitable for use in a wavelength multiplex optical transmission system.
2) Description of the Related Art
In the recent years, as one example of the WDM (Wavelength Division Multiplex) optical transmission systems, attention has been attracted with respect to a metrocore system which makes connections among local based cities and which is capable of carrying out the add/drop of optical signals with arbitrary wavelengths at arbitrary nodes. FIG. 13 is a block diagram showing an example of a configuration of a metrocore system. In this system shown in FIG. 13, a plurality of OADM (Optical Add-Drop Multiplexer) nodes 100 are connected through transmission lines (optical fibers) 400 into a ring-like configuration so that, at each of the OADM nodes 100, a signal light with an arbitrary wavelength (channel) can be added to the transmission line 400 and, of WDM signal light propagating along the transmission lines 400, a signal light with an arbitrary wavelength can be dropped therefrom. In addition, optical amplifiers (pre-amplifier 200 and post-amplifier 300) are respectively provided properly at former and latter stages relative to each of the OADM nodes 100 for making a compensation for loss of signal light level among the OADM nodes (which hereinafter will equally be referred to simply as “nodes”) to lengthen the transmission distance.
In a such a system designed to add/drop signal light at an arbitrary node, since the number of signal wavelengths (which will hereinafter be referred to equally as “number of transmission wavelengths”) transmitted in the system (transmission lines 400) varies dynamically, in order to maintain constant output optical power of each wavelength (channel) against this wavelength number variation (maintain the gain flat characteristic with respect to wavelength), an AGC amplifier having an automatic gain control (AGC) function is commonly used for each of the above-mentioned amplifiers 200 and 300.
In this case, for example, as shown in FIG. 14, the AGC amplifier is made such that portions of input/output lights to/from an optical amplifier (EDFA) 200 (300) are dropped by optical dropping means 501 and 502, respectively, and the respective powers [that is, input/output optical powers to/from the optical amplifier 200 (300)] are monitored by PDs 601 and 602, respectively, so that the excitation optical power to the EDFA 200 (300) is controlled by an automatic gain control unit 700 so as to make the power ratio constant.
Meanwhile, for example, as shown in FIG. 15(A), in such a system, let it be assumed that a large number of optical signals (for example, 39 wavelengths) are added from a given node 100 (100A) and one different one-wavelength optical signal is added from the next node 100 (100B). In this situation, for example, as shown in FIG. 15(B), in a case in which a trouble such as dynamic re-construction of an optical transmission path, man-made mistake, fiber disconnection or connector fall-out has occurred between the nodes 100A and 100B, only the signal light added at the node 100B remains (that is, the number of transmission wavelengths varies abruptly).
In response to this abrupt variation of the number of transmission wavelengths, for example, as shown in FIG. 16(A), there occurs a phenomenon that the power level of the residual optical signal varies at a signal reception end.
In this case, for example, as shown in FIG. 22, the aforesaid “signal reception end” signifies an optical receiver 101 having an optical/electrical conversion (O/E) function to receive a dropped light for converting it into an electric signal, and this also applies to the following description. Moreover, a “signal transmission end” signifies an optical transmitter 102 having an electrical/optical conversion function (E/O) to transmit a transmission signal (electric signal) with an added light having a predetermined wavelength.
For example, as shown in FIG. 16(B), the aforesaid signal optical power fluctuation stems mainly from three factors: (1) spectral hole burning (SHB), (2) gain (wavelength) deviation and (3) stimulated Raman scattering (SRS) effect. Each of these factors will be described hereinbelow.
(1) SHB
The first factor “SHB” is a phenomenon occurring in an optical amplifier 200 (300) and shows a feature that the shorter wavelength side optical signal power falls. That is, for example, as shown in FIG. 17, when an optical signal with one wavelength (for example, 1538 nm) in the C band (1530 to 1565 nm) is amplified in the optical amplifier 200 (300), there occurs a phenomenon that the EDFA gain in the vicinity of this signal wavelength drops (which is referred to as main hole) and the EDFA gain in the vicinity of 1530 nm also falls (which is referred to as second hole).
In addition, in the C band, there is a characteristic that the main hole becomes deeper toward the shorter wavelength side (the gain falling degree increases), and the main hole and the second hole become deeper as the optical signal input power increases. This SHB is subjected to the averaging in a state where a multi-wavelength signal light is inputted thereto and the influence thereof is low, whereas the influence thereof increases as the number of inputted wavelengths decreases. For this reason, for example, as shown in the row (1) of FIG. 16(B) and as shown in FIG. 18(A), in a case in which only one-wavelength signal light remains because a trouble has occurred between the nodes 100A and 100B as mentioned above, there occurs a phenomenon that the gain of the optical amplifier 200 (300) further decreases in the case of the shorter wavelength side residual signal light, which causes a reduction (−ΔP) of the output optical power.
(2) Gain Deviation
The second factor “gain (wavelength) deviation” is a phenomenon occurring in the optical amplifier 200 (300). That is, as mentioned above, the optical amplifier 200 (300) is designed to execute control (AGC) for maintaining a constant average gain of signal light and, when a wavelength producing a deviation remains, as shown in the row (2) of FIG. 16(B), with respect to the residual optical signal, the output optical power varies (in this case, +ΔP).
(3) SRS Effect
The third factor “SRS effect” is a phenomenon occurring in the transmission lines 400. The Raman amplifier is an optical amplifier utilizing this SRS effect. For example, as shown in FIG. 19, the SRS of a common single mode fiber shows a feature that a gain peak appears on a lower frequency side shifted by approximately 13 THz from the excitation light wavelength (longer wavelength side by approximately 100 nm in a case in which the excitation light wavelength is in the vicinity of 1400 nm), and the optical signal amplification in an arbitrary wavelength band becomes feasible by the selection of an excitation light wavelength. However, as shown in FIG. 19, difficulty is still experienced in enabling the amplification of a pinpoint wavelength, and the amplification (gain) characteristic has some degree of spread with respect to wavelength and, hence, the amplification phenomenon occurs even in the vicinity of the excitation light wavelength.
That is, in a case in which a WDM optical signal is transmitted through the transmission lines 400, the shorter wavelength side signal optical power becomes the excitation optical power, which amplifies the longer wavelength side signal light. In consequence, as shown in FIG. 20, the phenomenon appears that the signal optical power increases toward the longer wavelength side. Accordingly, in a case in which only one-wavelength signal light remains due to the occurrence of a trouble between the nodes 100A and 100B as mentioned above, as shown in the row (3) of FIG. 16(B) and in FIG. 18(B), the longer wavelength side residual signal light encounters a larger difficulty to take the power from the shorter wavelength side, and a power (gain) reduction (−ΔP) occurs.
Thus, if the number of wavelengths of a WDM signal light propagating through the transmission lines 400 varies largely, mainly, due to the three factors of the SHB, gain deviation and SRS, the output optical power of the residual signal light (residual channel) varies. In the OADM node 100, it is possible to place a function to adjust the levels of the signal lights with the respective wavelengths through the use of the feedback control, and this feedback control can cope with the fluctuation of the output optical power stemming from the aforesaid variation of the number of signal light wavelengths and the locations.
However, this respective-wavelengths handling feedback control usually takes a considerable long time from the variation of the number of wavelengths (time t0) up to the steady-state functioning (time t2) as shown in FIG. 16(A). That is, difficulty is actually encountered in suppressing even the transient fluctuation of the output optical power after the variation of the number of wavelengths as shown by the time t1 in FIG. 16(A).
In addition, with respect to such a transient fluctuation of the output optical level, although the fluctuation for each node 100 or for each transmission line 400 is not very large, the power fluctuation characteristics of one optical amplifier 102 and the transmission line 400 increases cumulatively in the case of the system in which similar optical amplifiers 102 carrying out the AGC and the transmission lines 400 are provided in a multi-stage fashion.
For example, as shown in FIG. 21(A), before the variation of the number of wavelengths, the reception level at a signal reception end (see the optical receiver 101 in FIG. 22) under each node 100 is in a normal range (reception tolerable range) even if the number of spans increases. That is, as mentioned above with reference to FIG. 15(A), in a case in which 39 waves are added at the node 100A while 1 wave is added at the node 100B and light having 40 waves in total from the nodes 100A and 100B is received at a signal reception end, in the case of a normal state where a trouble shown in FIG. 15(B) does not occur, even if a signal reception end is provided under any one of the nodes existing between the spans #1 to #5 in FIG. 15(A), the received optical power fluctuation is not accumulated at the signal reception end, so stable light reception becomes feasible.
However, when a variation in the number of wavelengths occurs as shown in FIG. 15(B), as shown in FIG. 21(B), the optical power fluctuation (ΔP) per channel which occurs in the respective optical amplifiers 200 and 300 and the transmission lines 400 increases cumulatively due to an increase in the number of spans, i.e., an increase in the number of passing optical amplifiers in a multi-stage fashion. That is, the received optical power fluctuation due to the influence of the variation of the number of wavelengths increases between the nodes 100A and 100B as a signal reception end is provided under the node 100 at a place where the number of passing spans #1 to #5 increases. The example in FIG. 21(B) shows that the negative-side reception power fluctuation increases due to an increase in number of spans.
In the case of a conventional optical transmission system which provides a short transmission distance and which has a small number of stages of the optical amplifiers, this fluctuation is minute and does not create a problem. However, in the future, along with a further increase in the number of stages of optical amplifiers for the long-distance transmission of the system, as shown in FIG. 21(B), there is a possibility that one of the optical signal powers of the respective wavelengths (channels) at the signal reception end or a plurality of optical signal powers are out of a reception tolerable range, which creates a transmission error. In other words, in a wavelength multiplex optical transmission system which performs a repeating transmission through a plurality of nodes 100 each having an OADM function, there is a possibility that the occurrence of a trouble in one transmission line zone can affect the communication in another transmission line zone.
Accordingly, considering the future development of a further lengthening of transmission distance, there is a need for realizing stable optical reception irrespective of the aforesaid occurrence of the optical power fluctuation.
As techniques related to the invention of the present application, for example, there are the techniques disclosed in the following Patent Documents 1 to 3.
The Patent Document 1 discloses an optical wavelength demultiplexer having a function to detect a level of an optical signal demultiplexed according to wavelength and further to attenuate the optical signal level of the corresponding wavelength as the detected optical signal level is higher.
Moreover, the Patent Document 2 discloses a technique of reducing the degradation of the SRS through the use of an amplifier for amplifying WDM channels simultaneously and a WDM shaping means.
Still moreover, the Patent Document 3 discloses a technique of suppression-controlling the transient fluctuation of the signal light level due to the SRS or SHB in a transmission apparatus at a repeating stage.    [Patent Document 1] Japanese Patent Laid-Open No. 2003-198478    [Patent Document 2] Japanese Patent Laid-Open No. HEI 9-8730    [Patent Document 3] Japanese Patent Laid-Open No. 2006-295113
However, each of the above-mentioned Patent Documents 1 to 3 does not disclose a configuration for carrying out the optical reception in an error-free condition even if received optical level fluctuation factors due to the above-mentioned variation of the number of wavelengths appear in transmission lines.